FIG. 14 is a sectional view illustrating a prior art buried heterostructure (BH) semiconductor laser. FIGS. 15(a)-15(e) are sectional views illustrating process steps in a method for fabricating the BH semiconductor laser of FIG. 14.
In the figures, a HB semiconductor laser 201 includes a stripe-shaped mesa structure 201a in which laser oscillation occurs and a light and current confinement structure 201b for confining laser light and laser driving current within the mesa structure 201a.
The mesa structure 201a comprises a mesa part 1a of an n type InP substrate 21a, an undoped active layer 2 disposed on the mesa part 1a, and a first p type InP cladding layer 3c disposed on the active layer 2. The light and current confinement structure 201b comprises an Fe doped semi-insulating InP layer 50 which is disposed on the n type InP substrate 21a contacting opposite sides of the mesa structure 201a and an n type InP layer 40 disposed on the semi-insulating InP layer 50.
A second p type InP cladding layer 3d is disposed on the mesa structure 201a as well as on the light and current confinement structure 201b. A p type InP contact layer 80 is disposed on the second p type InP cladding layer 3d. An insulating film 10 having a window 10a opposite the mesa structure 201a is disposed on the contact layer 80. A p side electrode 28 comprising AuZn alloy is disposed on the insulating film 10 contacting the p type InP contact layer 80 through the window 10a. An n side electrode 27 comprising CrAu alloy is disposed over the rear surface of the n type InP substrate 21a.
A description is given of the production process.
Initially, as illustrated in FIG. 15(a), an undoped InGaAsP layer 2a 0.1 .mu.m thick and a p type InP layer 3c.sub.1 0.5 .mu.m thick are successively grown on the n type InP substrate 21a(first crystal growth). The carrier concentration of the p type InP layer 3c.sub.1 is about 1.times.10.sup.18 cm.sup.31 3.
Then, an insulating film 31 of a prescribed pattern is formed on the p type InP layer 3c.sub.1 using conventional photolithographic techniques. Using the insulating film 31 as a mask, the p type InP layer 3c.sub.1, the undoped InGaAsP layer 2a, and the n type InP substrate 21a are selectively etched with a HBr system etchant to a depth of about 4.5 .mu.m from the surface of the p type InP layer 3c.sub.1, forming the stripe-shaped mesa structure 201a (FIG. 15(b)).
In the step of FIG. 15(c), the Fe doped semi-insulating InP layer 50 is grown on the n type InP substrate 21a contacting opposite sides of the mesa structure 201a, and the n type InP layer 40 is grown on the Fe doped semi-insulating InP layer 50 until the surface of the InP layer 40 reaches the surface of the mesa structure 201a(second crystal growth), forming the light and current confinement structure 201b. The thickness of the semi-insulating InP mesa embedding layer 50 on the flat surface of the substrate 21a is about 3 .mu.m, and the thickness of the n type InP layer 40 on the flat surface of the layer 50 is about 1.5 .mu.m. The carrier concentrations of these layers 50 and 40 are about 4.times.10.sup.16 cm.sup.-3 and 7.times.10.sup.18 cm.sup.-3, respectively.
In the step of FIG. 15(d), after removal of the insulating film 31, the second p type InP cladding layer 3d having a thickness of about 1 .mu.m and a carrier concentration of about 1.times.10.sup.18 cm.sup.-3 is formed on the first p type InP cladding layer 3c of the mesa 201a and on the n type InP layer 40 of the light and current confinement structure 201b. Then, the p type InP contact layer 80 having a thickness of about 0.5 .mu.m and a carrier concentration of about 7.times.10.sup.18 cm.sup.-3 is formed on the cladding layer 3d.
Thereafter, the insulating film 10 having the window 10a opposite the mesa 201a is formed on the p type InP contact layer 80. Then, the p side electrode 28 is formed in contact with the p type InP contact layer 80 at the window 10a, and the n side electrode 27 is formed on the rear surface of the n type InP substrate 21a, completing the BH semiconductor laser 201 (FIG. 15(e)).
A description is given of the operation.
When a prescribed voltage is applied across the p side electrode 28 and the n side electrode 27, a potential difference is produced between the n type InP substrate 21a and the second p type InP cladding layer 3d, and current flows from the p type InP cladding layer 3d to the InP substrate 21a.
Since the semi-insulating InP layer 50 and the n type InP layer 40 are disposed on the opposite sides of the stripe-shaped mesa structure 201a, current is concentrated in the mesa structure 201a, and holes and electrons (charge carriers) are injected into the active layer 2 with high efficiency. When the quantity of the injected charge carriers reaches a certain level, laser oscillation occurs.
Since the semi-insulating InP mesa embedding layer 50 comprises Fe-doped InP having a deep level that captures electrons, portions of the structure at the opposite sides of the active layer 2 have high resistivity, whereby reactive current that does not contribute the laser oscillation is reduced.
FIG. 16 is a sectional view illustrating a prior art BH semiconductor laser employing a p type InP substrate. FIGS. 17(a)-17(e) are sectional views illustrating process steps in a method for fabricating the BH laser of FIG. 16.
In FIG. 16, a BH semiconductor laser 202 includes a p type InP substrate 1 having a surface in or near a (001) plane. A p type InP lower cladding layer 1b having a stripe-shaped mesa portion 1b.sub.1 is disposed on the surface of the p type InP substrate 1. A stripe-shaped mesa structure 202a comprises the mesa portion 1b.sub.1 of the lower cladding layer 1b, an undoped InGaAsP active layer 2 disposed on the mesa portion 1b.sub.1, and an n type InP upper cladding layer 3a disposed on the active layer 2. The stripe direction of the mesa 202a is parallel to a (110) direction.
An n type InP current blocking layer 41 is disposed on the p type InP lower cladding layer 1b contacting the opposite sides of the mesa structure 202b. A pn junction barrier produced between the p type InP cladding layer 1b and the n type InP current blocking layer 41 blocks holes. An Fe-doped InP layer 51 that traps electrons is disposed on the n type InP current blocking layer 41. The surface of the Fe-doped InP layer 51 is even with the surface of the mesa structure 202a.
The n type InP current blocking layer 41 and the Fe-doped InP layer 51 form light and current confinement structure 202b for confining laser light and laser driving current in the active layer 2 of the mesa structure 202a.
A second n type InP cladding layer 3b is disposed on the mesa structure 202a as well as on the light and current confinement structure 202b, and an n type InP contact layer 70 is disposed on the n type InP cladding layer 3b.
A description is given of the production process.
Initially, a p type InP lower cladding layer 1b 2 .mu.m thick, an undoped InGaAsP layer 2a 0.1 .mu.m thick, and an n type InP layer 3a.sub.1 about 0.5 .mu.m thick are successively grown on the (001) or almost (001) surface of the p type InP substrate 1 by MOCVD (Metal Organic Chemical Vapor Deposition). Then, an SiO.sub.2 film 1000 .ANG. thick is deposited on the n type InP layer 3a.sub.1 by sputtering and patterned using conventional photolithographic techniques to form a stripe-shaped SiO.sub.2 pattern 32 along the (110) direction (FIG. 17(a)). The carrier concentrations of the p type InP lower cladding layer 1b and the n type InP layer 3a are 1.times.10.sup.18 cm.sup.-3.
Using the SiO.sub.2 pattern 32 as a mask, the n type InP layer 3a.sub.1, the InGaAsP layer 2a, and the p type InP lower cladding layer 1b are selectively etched with a HBr system etchant to a depth of 2.5 .mu.m from the surface of the n type InP layer 3a .sub.1, forming the stripe-shaped mesa structure 202a (FIG. 17(b)).
Then, the n type InP current blocking layer 41 having a thickness of about 1 .mu.m and a carrier concentration of about 7.times.10.sup.18 cm.sup.-3 is formed on the p type InP lower cladding layer 1b contacting the opposite sides of the mesa structure 202a, and the Fe-doped InP high resistivity layer 51 1.5 .mu.m thick having a carrier concentration of 4.times.10.sup.16 cm.sup.-3 is formed on the current blocking layer 41 (FIG. 17(c)). These layers 41 and 51 are grown by MOCVD. In the MOCVD process, the n type InP current blocking layer 41 grows contacting the opposite side surfaces of the mesa structure 202a, and a surface in a (111)B plane appears at the upper end 41a of the current blocking layer 41 under the SiO.sub.2 film 32 and a surface in a (221)B plane appears at the side surface 41b of the layer 41 along the side surface of the mesa.
After etching of the SiO.sub.2 film 32 with HF, the second n type InP cladding layer 3b about 1 .mu.m thick having a carrier concentration of 1.times.10.sup.18 cm.sup.-3 is grown on the first n type InP cladding layer 3a and on the InP high resistivity layer 51 and, successively, the n type InP contact layer 70 about 0.5 .mu.m thick with a carrier concentration of 7.times.10.sup.18 cm.sup.-3 is grown on the cladding layer 3b by MOCVD (FIG. 17(d)).
Thereafter, the insulating film 10 having the window 10a opposite the mesa 202a is formed on the second n type InP cladding layer 3b. Finally, the n side electrode 7 is formed contacting the n type InP contact layer 70 through the window 10a, and the p side electrode 8 is formed on the rear surface of the p type InP substrate 1, completing the BH semiconductor laser 202 (FIG. 17(e)).
A description is given of the operation.
The operation of this HB laser 202 for laser oscillation is identical to that already described with respect to the HB laser 201 of FIG. 14. Hereinafter, the function of the light and current confinement structure 202a comprising the n type InP current blocking layer 41 and the Fe-doped InP high resistivity layer 51 will be described.
FIGS. 22(a) and 22(b) are energy band diagrams of the light and current confinement structure 202b, i.e., the laminated structure comprising the p type InP cladding layer 1b, the n type InP current blocking layer 41, the Fe-doped InP high resistivity layer 51, and the second n type InP cladding layer 3b, in which FIG. 22(a) illustrates the energy band in a thermal equilibrium state where no bias is applied and FIG. 22(b) illustrates the energy band in a state where a forward bias is applied (hereinafter referred to as forward biased state). FIGS. 21(a) and 21(b) are energy band diagrams of a light and current confinement structure comprising only the Fe-doped InP high resistivity layer 51, in the thermal equilibrium state (FIG. 21(a)) and in the forward biased state (FIG. 21(b)). FIGS. 20(a) and 20(b) are energy band diagrams of an ordinary pn junction in the thermal equilibrium state (FIG. 20(a)) and in the forward biased state (FIG. 20(b)). In these figures, reference character InDE denotes a diffusion current of electrons, InDRa drift current of electrons, IpDE a diffusion current of holes, and IpDR a drift current of holes.
In the BH semiconductor laser, if the light and current confinement region disposed on the opposite sides of the stripe-shaped mesa comprises only the Fe-doped InP high resistivity layer 51 as shown in FIGS. 21(a)-21(b), when a forward bias V for laser oscillation is applied, the Fe-doped InP high resistivity layer 51 serves as a high resistivity current blocking layer that traps electrons, but holes cross a low energy barrier between the Fe-doped InP layer and the underlying p type InP cladding layer and are injected into the Fe-doped InP layer 51 (FIG. 21(b)). The holes recombine with the electrons in the Fe-doped InP layer 51, producing a recombination current. As a result, the Fe-doped InP high resistivity layer 51 loses its current blocking effect.
On the other hand, in the BH semiconductor laser 202 shown in FIG. 16 in which the light and current confinement structure 202b comprises the n type InP current blocking layer 41 and the Fe-doped InP high resistivity layer 51, when a forward bias V is applied across the p side electrode 8 and the n side electrode 7, the height of the pn junction barrier between the p type InP lower cladding layer 1b and the overlying n type InP current blocking layer 41 does not change because the energy band inclines at the Fe-doped InP high resistivity layer 51 and the applied forward bias V is applied to that layer 51.
Therefore, the injection of holes from the p type InP cladding layer 1b to the Fe-doped inP high resistivity layer 51 is prevented, whereby the laser driving current is blocked by the light and current confinement structure 202b and supplied with high efficiency to the active layer 2 in the mesa structure 202a sandwiched by the current confinement structure 202b.
FIG. 18 is a sectional view illustrating a prior art BH semiconductor laser including a light and current confinement structure including a pnpn thyristor. FIGS. 19(a)-19(e) are sectional views illustrating process steps in a method for fabricating the BH laser of FIG. 18.
In FIG. 18, a BH semiconductor laser 203 includes a pnpn thyristor light and current confinement structure 203b disposed on opposite sides of a stripe-shaped mesa structure 202a. The light and current confinement structure 203b comprises a p type InP mesa embedding layer 61 disposed on the p type InP cladding layer 1b, an n type InP current blocking layer 62 disposed on the mesa embedding layer 61, and a p type InP current blocking layer 63 disposed on the n type InP current blocking layer 62. Other elements are identical to those of the BH laser 202 of FIG. 16.
A description is given of the production process.
The steps illustrated in FIGS. 19(a) and 19(b) are identical to those already described with respect to FIGS. 17(a) and 17(b) and, therefore, repeated description is not necessary.
After formation of the stripe-shaped mesa structure 202a (FIG. 19(b)), the p type inP mesa embedding layer 61 about 0.7 .mu.m thick, the n type InP current blocking layer 62 about 0.8 .mu.m thick, and the p type InP current blocking layer 63 about 1 .mu.m thick are successively grown on the p type InP lower cladding layer 1b at opposite sides of the mesa structure 202a (FIG. 19(c)). These layers are grown by MOCVD. The carrier concentrations of the p type InP mesa embedding layer 61 and the p type InP current blocking layer 63 are about 1.times.10.sup.18 cm.sup.-3 and the carrier concentration of the n type InP current blocking layer 62 is about 7.times.10.sup.18 cm.sup.-3.
Since the opposite side surfaces of the mesa structure 202a are covered with the p type InP mesa embedding layer 61, the n type InP cladding layer 3a in the mesa structure is electrically separated from the n type InP current blocking layer 62 by the mesa embedding layer 61. The growth of the mesa embedding layer 61 proceeds, forming upper end surfaces 61a in (111)B planes under the SiO.sub.2 film 32 and side surfaces 61b in (221)B planes along the side surfaces of the mesa structure.
After removal of the SiO.sub.2 film 32 with HF, the second n type InP cladding layer 3b about 1 .mu.m thick is grown on the first n type InP cladding layer 3a and on the p type InP current blocking layer 63 and, successively, the n type InP contact layer 70 about 0.5 .mu.m thick is grown on the second n type InP cladding layer 3b (FIG. 19(d)). These layers are grown by MOCVD.
Thereafter, the insulating film 10 having the window 10a opposite the mesa structure 202a is formed on the n type InP contact layer 70. Finally, the n side electrode 7 is formed contacting the contact layer 70 through the opening 10a and the p side electrode 8 is formed on the rear surface of the p type InP substrate 1, comprising the BH semiconductor laser 203 with the pnpn thyristor light and current confinement structure 203b (FIG. 19(e)).
However, the above-described prior art semiconductor lasers 201 to 203 have the following drawbacks.
In the semiconductor laser 201 shown in FIG. 14, since the Fe-doped semi-insulating InP layer 50 is in contact with the active layer 2 whose conductivity type is changed to p type due to the diffusion of Zn from the p type InP cladding layer 3c, Fe in the InP layer 50 diffuses into the active layer 2 and contaminates the active layer 2, adversely affecting the characteristics of the semiconductor laser. The diffusion length of Fe is about 10 .mu.m while the diffusion length of Zn in the p type InP layer 3d is only 0.3 .mu.m.
Generally, it is difficult to make an ohmic contact between a p type InP layer and a p side electrode. Furthermore, in a semiconductor laser employing an n type InP substrate, the contact area of the p type InP layer and the p side electrode is reduced to reduce the capacitance of the device and increase operating speed, whereby the contact resistance of the p side electrode is increased and the resistance of the whole element is increased, adversely affecting the characteristics of the laser element.
In the semiconductor laser 202 shown in FIG. 16, since the n type InP current blocking layer 41 is in contact with the n type InP cladding layer 3a, reactive current Ir which does not contribute to the laser oscillation flows as shown in FIG. 16, whereby current is not injected into the undoped active layer 2 with high efficiency.
In the semiconductor laser 203 shown in FIG. 18, reactive current Ir flows from the p type InP mesa embedding layer 61 to the n type InP cladding layer 3b. Furthermore, as shown in FIG. 9(a), a depant impurity (Si) accumulates on the side surface of the stripe-shaped mesa 202a shown in FIG. 19(b), and current flows at the side of the undoped active layer 2 through the region including the accumulated dopant impurity. As a result, current is not injected into the undoped active layer 2 with high efficiency.
As described above, in the prior art semiconductor lasers 202 and 203 shown in FIGS. 16 and 18, respectively, since the reactive current flows at the side of the active layer, current is not injected into the active layer with high efficiency, whereby the characteristics and the reliability of the semiconductor laser are degraded.